manufacturer's suggestion to connect chain hoist to chain hoist

Each hoist is rated to safely lift and support a specified load from an attachment point that can also safely support both that load and the weight of the hoist itself. If that attachment point is another chain hoist it makes no difference as long as that second hoist can support the weight of the load and the first hoist, and the attachment point for the second hoist can support the load and the weight of both hoists. As for a shackle, if the hooks from the two hoists will connect properly there is no need for a shackle.

Is it safe to operate the upper chain fall from a ladder with the load swinging next to it?

If you have a properly rated shackle or welded ring (rated) even if both ends of the hoists have Slip Hooks, I'd still use it.

If one or the other has Grab Hooks (narrow slot meant to grab a chain) you'll need a chunk of rated chain to couple them up - or change it to a Slip Hook.

Because you won't get a Grab Hook to both mate and safely lock to a Slip Hook or a Shackle - putting the load hanging off of the tip of the hook instead of at the root of the hook is a recipe for disaster.

Not particularly safe, but then again there are no guarantees in life.

But if you can stand on a Scaffold or an A-ladder that has legs far enough out of the Splat Zone so the ladder won't get wiped out from under you if the load falls and bounces a few times, you should be OK. Or rig up a safety rope from above and wear a body harness. Might get Interesting, but should avoid fatalities or major injuries.